Surfaces of individual lenses, which construct an optical system such as a camera lens and an objective lens of a microscope, are coated with an antireflection film in order to reduce the reflection. In general, the optical thin film such as the antireflection film is produced by the dry method (dry process), in which the vacuum vapor deposition method, the sputtering method, the CVD method (Chemical Vapor Deposition) or the like is used.
In order to obtain a high performance optical thin film having a low reflectance in a wide wavelength band or a wide angle band, it is known that a multilayered film is appropriately formed by combining a plurality of coating materials having different refractive indexes. Usually, when an antireflection film is formed by the dry process, then TiO2 (refractive index: 2.4 to 2.7 at 500 nm) is usually utilized as the maximum refractive index material, and MgF2 (refractive index: 1.38 at 500 nm) is utilized as the minimum refractive index material.
The following fact is known for the multilayered film. That is, when the difference in the refractive index between the coating materials to be used is larger, or when a low refractive index film is used at the uppermost layer, then the optical performance is improved, and/or the number of coating layers can be decreased even when the optical performance is same. In particular, it has been clarified by the simulation that the optical performance can be extremely enhanced, when only the uppermost layer is composed of a low refractive index film having a refractive index of not more than 1.30. That is, the low refractive index film, in which the uppermost layer has a refractive index of not more than 1.30, is effective for the realization of the wide band in which the reflectance can be suppressed to be low over a wide wavelength region. Further, the low refractive index film is also extremely effective for the realization of the wide incidence in which the reflectance can be suppressed to be low not only for the direct or normal incoming (incident) light but also for the light allowed to come in a wide angle range. Therefore, a technique is required, in which the optical thin film having a refractive index of not more than 1.30 can be produced.
In order to lower the refractive index of the film, it is effective that the structure of the film is porous rather than dense. In general, it is defined that the film has a structure of a plurality of minute holes or pores which separate the deposited solid substance. Therefore, the relationship between the packing density and the refractive index of the film is as follows.nf=no×P+np×(1−P)
In this expression, np represents the refractive index of the substance (for example, air or water) with which the minute holes are filled, nf and no represent the actual refractive index (depending on the packing density) and the refractive index of the deposited solid material respectively, and P represents the packing density of the film. Further, the packing density is defined as follows.P=(volume of solid portion of film)/(total volume of film (solid portion+minute hole portion))
Thus, the high and low the packing densities mean the high and low the refractive indexes respectively.
In general, the dry process such as the vapor deposition and the sputtering is suitable in order to obtain the dense film. However, the wet method (wet process) is suitable in order to obtain the porous film. The wet process is such a method that the film is formed by coating the substrate with the liquid by, for example, the spin coat method, the dip method, the spray method, and the roll coat method, followed by being dried and heat-treated. The feature of the wet process is exemplified such that any large-sized apparatus is not required, unlike the dry process, and that the film can be formed in the atmospheric air. Therefore, it is possible to greatly lower the cost. For example, in the case of the lens having a small radius of curvature, it is difficult to uniformly effect the coating of the optical thin film by the dry process such as the vacuum vapor deposition method and the sputtering method. However, the uniform coating can be performed relatively easily in the case of the wet process such as the spin coat method. In this case, the film can be formed uniformly on a surface having a large areal size and on a curved surface having a small radius of curvature as well.
International Publication No. 02/18982A1 discloses a method for producing a porous MgF2 film by the wet process. In this method, a sol solution of MgF2 is heat-treated at a high temperature and a high pressure to thereby effect the grain growth and the crystallization of MgF2 minute particles, which is thereafter subjected to the coating to form the film. According to this method, even when the film is formed by depositing the MgF2 minute particles, the pores, which exist between the minute particles, are not crushed, and the high porosity is secured. As a result, the film is porous. It is possible to extremely lower the refractive index as compared with any dense film produced by the dry process. However, the following problem arises. That is, the mechanical strength of the obtained porous film is low, and the adhesive force is low with respect to the substrate, and that when the manual wiping is performed, the porous film is exfoliated.
A large number of techniques are known in order to improve the film strength and the adhesive force of the porous films based on the use of various types of minute particles. For example, Japanese Patent No. 3272111 discloses a technique for reinforcing or enhancing an antistatic film composed of SnO2 minute particles with which a surface of a cathode ray tube is coated. In this technique, a sufficient strength is given to an SnO2 film by forming an SiO2 film on the SnO2 film by the wet process. However, the refractive index is not lowered sufficiently, because the dense SnO2 film is formed at the uppermost layer.
A technique is disclosed in Japanese Patent Application Laid-open No. 11-6902 as an example of the techniques for reinforcing a porous film itself, in which the porous film composed of inorganic minute particles is reinforced with a polymer binder. In this technique, it is possible to reinforce the film itself. However, the refractive index of the film cannot be lowered to be not more than 1.30, because the refractive index of the polymer is relatively high.
Japanese Patent Application Laid-open Nos. 7-48527 and 8-122501 disclose a technique in which a porous film composed of SiO2 minute particles is reinforced with a binder of alkoxysilane. The film itself can be also reinforced in the case of this technique. However, SiO2 has a property to easily adsorb the water content in the air. Further, the film is porous, which has a large surface area. For this reason, the large wavelength shift is caused. Therefore, the film can be used as an antireflection film for display devices. However, it is difficult to use the film for any precision optical instrument such as cameras, microscopes and the like.
There is such a possibility that the wavelength shift can be suppressed by using a sol described, for example, in Japanese Patent Application Laid-open No. 2000-169133. This document describes, as a coating agent, the sol of composite colloid particles of 5 to 50 nm in which colloidal silica and MgF2 hydrate are coagulated. Although there is no description about the film in Japanese Patent Application Laid-open No. 2000-169133, when any film is formed, the wavelength shift is hardly caused because MgF2 has a property to hardly adsorb the water content. However, in the case of the sol as described above, it is not necessarily affirmed that the sol is excellent in the environment resistance, because the sol contains the unstable MgF2 hydrate which is not pure MgF2.
In recent years, the optical system is increasingly complex and versatile, as the required performance is enhanced. For example, the number of lenses is increased, for example, in order to maximally chase the aberration to the limit or in order to increase the zoom magnification. It is also necessary to provide such a design that the angle of incidence of the light beam into the lens surface is increased. Further, as the digital camera comes to the front in recent years, for example, the element, which has been the film, is progressively replaced with the image pickup device such as CCD and CMOS.
When the change of the optical system is assessed from a viewpoint of the surface reflection of the lens or the like, the increase in the number of lenses is directly the increase in the number of reflecting surfaces. The antireflection film is applied in ordinary cases. However, the possibility is increased that the ghost and flare are caused due to the residual reflection, and the transmittance is lowered as well. As for the increase in the angle of incidence, in principle, there is such a tendency that the surface reflection is increased as the incidence is effected more obliquely, irrelevant to the presence or absence of the antireflection film, which makes the cause of the ghost and the flare. The reflection of the image pickup device has not been hitherto considered. It is pointed out that the reflected light is returned to the optical system to cause the flare and the ghost. The ghost and the flare cause the decrease in the contrast and the deterioration of the color tone, and they cause the disappearance of the image in the worst case, which are of course unfavorable.
The antireflection film, which is generally used at present, is initially a singlelayered antireflection film. However, the singlelayered antireflection film is changed to the multilayered antireflection film in order to widen the wavelength band or zone. As the production technique is improved, the antireflection film is sufficiently investigated and contrived, for example, such that the antireflection characteristic is adjusted. The optical design is progressively contrived as well, for example, such that the angle of incidence is restricted or limited so that various problems are not caused, in consideration of the proper arrangement of the antireflection film. As a result, a lens, which involves less problems to some extent, is completed (see Japanese Patent Application Laid-open No. 62-124503).
However, such a lens is established on the sacrifice of the degree of freedom of the optical design. As the high performance is required and/or the new element or device such as CCD is used as described above, it is recognized that the performance of the conventional antireflection film is insufficient.